Allocate and define AF_ARP and PF_ARP. These will be used to
a) communicate the hardware independent part of ARP packets (rfc826) from/to
if_*subr.c in the ongoing effort to rewrite the ARP subsystem for non-Ethernet
networks
b) communicate the hardware independent part of RARP packets (rfc903) from/to
userland sockets to avoid putting link level header format knowledge into
rarpd, when it is enhanced to support non-Ethernet networks.